During the starting phase of an elevator run in case of an unbalance between the weight of the car and the counterweight, the starting torque of the motor has to be set in a way to avoid sagging and to match reference values of jerk and acceleration.
Sagging of the elevator during starting is usually avoided by the use of one of two techniques:
1. With passenger load information:
setting the motor torque equal to the load torque before opening the brake according to the load information coming from a load sensor. PA2 activating a velocity dictation profile before opening the brake, to produce a motor torque which relates to the load, thus pulling the elevator out of the brake.
2. Without load information:
Technique 1 requires a load sensor which increases the costs of the system.
Technique 2 is cost effective, but produces a start jerk and acceleration overshoot due to the following principal reasons.
To avoid sagging, the overlapping of the brake release and the velocity profile have to be adjusted for the worst case starting condition which is full load up. The starting motor torque demand, i.e., the velocity regulator output, is produced due to a tracking error between dictated and actual velocity. Due to the operation principle of the velocity regulator, this start tracking error will be reduced during the acceleration phase. This is done by increasing the acceleration and its slope, i.e, the jerk, until the dictated profile can be tracked.
In the case of an empty car, the torque produced in the motor when opening the brake is much too high. This will additionally increase the start jerk.